The invention relates to a method of down-converting a high-frequency signal, and a down conversion mixer and especially a quadrature down conversion mixer for carrying out the method. The invention furthermore relates to a phone having such a down conversion.
In a traditional super heterodyne receiver, non-linearities of second order will result in mixer products that are far away in frequency from the IF signal at the output. These undesired signals can therefore be removed by the IF filter. The dominant non-linearities in the conventional super heterodyne receivers are therefore the 3rd order intermodulation products.
In a direct conversion receiver, however, mixer non-linearities of second order will result in the down conversion of any signal, presented to the mixer input, to a baseband signal. A pure carrier will thus give rise to a DC signal at the output, while an AM modulated signal will result in a low-pass baseband signal whose spectral contents might overlap the desired signal.
High suppression of second order products in the mixer is therefore of great importance, and lack of sufficient second order suppression has often been one of the reasons why the direct conversion receiver has been discarded as a realistic alternative to the conventional super heterodyne receiver.
Generally, the manufacturers of the mixers are able to suppress the second part of the expression (8) to a level which is 40-50 dB below the level of the first part of the expression. However, in GSM phase II an AM rejection to at least 70 dB is required.